The purpose of this research program is to identify the location and function of gap junctions in the mammalian retina. This will build directly on work in the previous grant period in which we identified a horizontal cell connexin in the mammalian retina for the first time and developed a slice preparation of the rabbit retina for physiological experiments. Specific Aim 1. Horizontal Cell Gap Junctions. We will test the hypothesis that Cx50 gap junctions are responsible for coupling in A-type horizontal cells. We will determine if Cx50 is expressed in other cell types and if the Cx50 staining pattern is consistent with the distribution of gap junctions in the A-type horizontal cell network. We will look for the presence of Cx50 hemichannels in horizontal cell dendrites at the cone pedicle. Finally, we will determine if Cx50 is expressed in the horizontal cells of rodent, cat and primate retina. Specific Aim 2. Gap Junction Blockers. A potent, specific, soluble and reversible blocker of gap junction coupling is required for the functional analysis of retinal circuitry. We will test several novel compounds to see if they block gap junctions in the retina. We will identify the most potent and reversible of these compounds and test their specificity on several cell types which form coupled networks, including A- and B- type horizontal cells and AII amacrine cells. By testing coupling in different networks, we will test the hypothesis that certain compounds can block gap junctions in the retina with some selectivity for different connexins. Specific Aim 3. The function of gap junctions in the rod pathway. We have developed methods by which rod bipolar cells, AII amacrine cells and ON cone bipolar cells can be identified and recorded in slices of rabbit retina. The light response of AII amacrine cells and ON cone bipolar cells is derived from several different sources which mix according to the light intensity. By identifying or blocking these different components, we will test the hypothesis that increasing intensity generates rod signals via the rod bipolar cell, then rod signals via rod/cone coupling and, finally, cone signals via ON cone bipolar cells.